tiptap: fix focus issue when editor not mounted (tldraw#7043)

tiptap: fix focus issue when editor not mounted

### Change type

- [x] `bugfix`
- [ ] `improvement`
- [ ] `feature`
- [ ] `api`
- [ ] `other`

### Release notes

- tiptap: fix minor focus issue if editor isn't mounted yet

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Guard Tiptap toolbar actions and active-state checks when the editor
view isn't mounted to prevent focus errors.
> 
> - **Rich Text Toolbar (`DefaultRichTextToolbarContent.tsx`)**:
> - Add guard to avoid invoking Tiptap operations when `textEditor.view`
is undefined.
> - Compute `isActive` defensively using `textEditor.view ? ... : false`
to prevent access before mount.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
299507f. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: mimecuvalo

packages/tldraw/src/lib/ui/components/Toolbar/DefaultRichTextToolbarContent.tsx

@@ -54,6 +54,9 @@ export function DefaultRichTextToolbarContent({
 	// todo: we could make this a prop
 	const actions = useMemo(() => {
 		function handleOp(name: string, op: string) {
+			// Check if the editor view is available before calling operations
+			if (!textEditor.view) return
+
 			trackEvent('rich-text', { operation: name as any, source })
 			// @ts-expect-error typing this is annoying at the moment.
 			textEditor.chain().focus()[op]().run()
@@ -109,7 +112,7 @@ export function DefaultRichTextToolbarContent({
 	}, [textEditor, trackEvent, onEditLinkStart])
 
 	return actions.map(({ name, attrs, onSelect }) => {
-		const isActive = textEditor.isActive(name, attrs)
+		const isActive = textEditor.view ? textEditor.isActive(name, attrs) : false
 		return (
 			<TldrawUiToolbarButton
 				key={name}